1. Field of the Invention
This invention relates to semiconductor interband lasers and more particularly, to semiconductor mid-infrared diode lasers having improved thermal dissipation and being cable of operation in longer wavelength spectrum.
2. Brief Description of Related Art
Semiconductor lasers have been developed with emission wavelengths (λ) ranging from near- to mid-infrared (λ>3 μm) and beyond. When the wavelength of a semiconductor laser is long, the cladding layer thickness of the waveguide structure for the laser must be made thicker. For instance, mid-infrared interband cascade (IC) lasers typically use 2-3 μm-thick InAs/AlSb superlattice (SL) as the cladding layer to confine the optical wave in a waveguide. See, e.g., Yang, “Mid-Infrared Interband Cascade Lasers Based on Type-II Heterostructures”, Microelectronics J. Vol. 30, 1043 (1999); Hill, et al, “MBE Growth Optimization of Sb-Based Interband Cascade Lasers”, J. Crystal Growth vol. 278, 167 (2005); Vurgaftman, et al, “Mid-infrared interband cascade lasers operating at ambient temperatures”, New J. Phys. Vol. 11, 125015 (2009). The use of thick InAs/AlSb SL cladding layers in IC lasers is very demanding for growth by molecular beam epitaxy (MBE) with so many shutter movements. Furthermore, an InAs/AlSb SL layer has a very low thermal conductivity (κ˜ 0.03 W/cm·K) as indicated by Borca-Tasciuc, et al. in the paper entitled “Thermal conductivity of InAs/AlSb superlattices” published in Microscale Thermophys. Eng. Vol. 5, 225 (2001), and thick SL layers cause significant heating. Because the SL cladding layer has a refractive index (˜3.37) that is only slightly smaller than that of the cascade region (3.43 to 3.47), its thickness cannot be reduced, which could lead to substantial leaking of the optical wave into the GaSb substrate (refractive index ˜3.8), resulting in undesirable optical loss. This situation will become worse if SL cladding layers are still used in IC lasers for longer wavelengths because of the requirement of thicker cladding layers. Hence, if the SL cladding layer can be replaced with appropriate material, IC laser performance will improve significantly. Also, it is feasible to extend efficient IC lasers to longer wavelengths.
Therefore, it is an object of the present disclsoure to provide an improved semiconductor mid-infrared diode laser with greater thermal dissipation and being adapted to generate or emit light having a lasing wavelength longer than lasing wavelengths using previous IC lasers.